Use Google Calendar, then press (or have your clients) "create" which will create a new event. You can setup calendar sharing with specific users, as long as you have their email.

Tilman

October 20, 2011 at 3:48 pm

I second Google Calendar. A new feature is "Appointment slots" which work a bit like a real appointment manager :) Oh and you don't have to know the emails of your customers necessarily if you make your calendar public. Even in that case you can still decide what the public can see, whether they can see all details or just whether you are busy or not.